Degradation-determination system and method for determining degradation of secondary battery

ABSTRACT

A degradation-determination system includes at least four strain gauges that are installed on a principal surface of a lithium-ion battery and each of which is configured to detect pressure of a battery surface at a corresponding installation position, and a degradation determining unit configured to determine degradation of the lithium-ion battery based on measured values at the strain gauges. The degradation determining unit is configured to estimate a maximum expansion position where volume expansion is maximal in a region defined by the strain gauges, of the surface of the lithium-ion battery.

TECHNICAL FIELD

The present disclosure relates to a degradation-determination system anda method for determining degradation of a secondary battery.

BACKGROUND

Secondary batteries such as lithium-ion batteries have higher energydensity, and are compact and lightweight. For this reason, suchsecondary batteries are widely used in electricity storage systems suchas electric vehicles or smartphones.

Repeated charging and discharging causes lithium-ion batteries todegrade. In the past, an approach for determining degradation of abattery by measuring pressure of a principal surface of the battery hasbeen proposed (for example, Patent document 1).

CITATION LIST Patent Document

[Patent document 1] Japanese Unexamined Patent Application PublicationNo. 2018-81854

SUMMARY

For the conventional approach for determining degradation described inPatent document 1 or the like, variations in pressure of the wholeprincipal surface of a lithium-ion battery are monitored, and it is onlydetermined whether degradation of the battery occurs. However, forpurposes of improvement or the like of the safety of the battery, thereis a need to identify a position where volume expansion of the principalsurface is maximal to thereby locally identify a position of degradationof the battery.

An objective of the present disclosure is to provide adegradation-determination system and a method for determiningdegradation of a secondary battery that are capable of locallyidentifying a position of degradation of a battery.

According to one aspect of one or more embodiments of the presentdisclosure, a degradation-determination system for a secondary batteryincludes at least four pressure detecting units that are installed onone or more surfaces of the secondary battery and each of which isconfigured to detect pressure of a battery surface at a correspondinginstallation position, and a degradation determining unit configured todetermine degradation of the secondary battery based on measured valuesat the at least four pressure detecting units. The degradationdetermining unit is configured to estimate a volume expansion positionwhere volume expansion is maximal in a region defined by the at leastfour pressure detecting units, of the one or more surfaces of thesecondary battery.

Likewise, according to one aspect of one or more embodiments of thepresent disclosure, a method for determining degradation of a secondarybattery includes a pressure-detection step of detecting pressure of abattery surface at a corresponding installation position, by each of atleast four pressure detecting units installed on one or more surfaces ofthe secondary battery, and a degradation-determination step ofdetermining degradation of the secondary battery based on measuredvalues at the at least four pressure detecting units. In thedegradation-determination step, a volume expansion position where volumeexpansion is maximal in a region defined by the at least four pressuredetecting units, of the one or more surfaces of the secondary battery,is estimated.

Effects of the Invention

According to the present disclosure, a degradation-determination systemand a method for determining degradation of a secondary battery that arecapable of locally identifying a position of degradation of a battery,can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically illustrating the configurationof a degradation-determination system according to an embodiment;

FIG. 2 is a schematic diagram for describing an approach for estimatinga local position of degradation of a lithium-ion battery; and

FIG. 3 is a flowchart illustrating the procedure for adegradation-determination process of the lithium-ion battery.

DESCRIPTION OF EMBODIMENTS

One or more embodiments will be hereafter described with reference tothe accompanying drawings. In order to facilitate the understanding ofexplanation, in each figure, the same numerals denote the samecomponents to the extent possible, and duplicative descriptions for thecomponents will be omitted.

FIG. 1 is a block diagram schematically illustrating the configurationof a degradation-determination system 1 according to the embodiment.FIG. 2 is a schematic diagram for describing an approach for estimatinga local position G of degradation of a lithium-ion battery 2. Thedegradation-determination system 1 determines degradation of thelithium-ion battery 2 as an example of a secondary battery. Asillustrated in FIG. 1, the degradation-determination system 1 includes acharging device 3, a controller 4, and strain gauges 5A to 5D (pressuredetecting units).

The lithium-ion battery 2 has the configuration illustrated in theexample in FIG. 1, and is coated with a thin, substantially cuboidalhousing 21 having a pair of principal surfaces 24. In FIG. 1, the pairof principal surfaces 24 of the housing 21 is disposed so as to faceeach other along a depth direction in the figure. Each principal surfaceis approximately rectangularly shaped, and a positive terminal 22 and anegative terminal 23 are provided on one (upper surface in FIG. 1) amongfour side surfaces of the housing 21 that are each perpendicular to theprincipal surface 24. One end of each of the positive terminal 22 andthe negative terminal 23 protrudes outward from the housing 21, and isconnected to the charging device 3. The lithium-ion battery 2 may be asingle cell as illustrated in FIG. 1, or an assembled battery in which aplurality of single cells as illustrated in FIG. 1 are connectedtogether.

The charging device 3 is connected to the positive terminal 22 and thenegative terminal 23 of the lithium-ion battery 2, and charges thelithium-ion battery 2 via the positive terminal 22 and the negativeterminal 23. For example, the charging device 3 stores a set valueindicative of an upper limit (factor of safety) of a charge tolerancethat corresponds to the extent of degradation of the battery. Thecharging device 3 can charge the battery to the upper limit whilereferring the remaining battery capacity. The charging device 3 alsooutputs data, such as a time required to fully charge the battery, tothe controller 4.

The controller 4 controls charging through the charging device 3. Thecontroller 4 also estimates a local position G (see FIG. 2) ofdegradation of the lithium-ion battery 2, based on information inputfrom the strain gauges 5A to 5D. For these related functions, thecontroller 4 has a charge control unit 41 and a degradation determiningunit 42.

The charge control unit 41 controls the charging process of thelithium-ion battery 2 by the charging device 3. The charge control unit41 adjusts the charging time or the voltage value. In order for thebattery to operate more stably, the charge control unit 41 appropriatelyadjusts one or more parameters related with the charge of thelithium-ion battery 2, based on the position G of degradation estimatedby the degradation determining unit 42, and then may output theparameters to the charging device 3.

The degradation determining unit 42 estimates the local position G ofdegradation of the lithium-ion battery 2, based on measured values S1 toS4 at the strain gauges 5A to 5D. The local position G of degradationrefers to a local portion of the principal surface 24 of the lithium-ionbattery 2 where the largest degradation occurs. The degradationdetermining unit 42 estimates a maximum expansion position G wherevolume expansion is maximal in a region defined by the strain gauges 5Ato 5D, in the principal surface 24 of the lithium-ion battery 2, andthen outputs the maximum expansion position G as the local position G ofdegradation. A specific approach for estimating the position G ofdegradation will be described below.

The controller 4 may be implemented by any hardware, software, or acombination thereof. The controller 4 may be mainly comprised of amicrocomputer including, for example, a central processing unit (CPU), arandom access memory (RAM), a read only memory (ROM), an auxiliarystorage device, an input-output interface (I/O), and the like. Thevarious functions described above are implemented by executing variousprograms, which are stored in the ROM, the auxiliary storage device, orthe like, on the CPU.

Each of the strain gauges 5A to 5D is installed on the surface of thelithium-ion battery 2 and outputs a given electrical signal amongelectrical signals S1 to S4, in accordance with strain of acorresponding installation portion, as illustrated in FIG. 2. As each ofthe strain gauges 5A to 5D, any type of strain gauge, such as a metalstrain gauge or a semiconductor strain gauge, may be adopted.

The strain gauges 5A to 5D are respectively installed at four cornerregions 24A to 24D (hereafter also denoted as corners 24A to 24D) of theprincipal surface 24 (surface) of the cuboidal lithium-ion battery 2, asillustrated in an example in FIG. 2. In the present embodiment, thestrain gauge 5A is installed at the corner 24A that is on the side ofthe negative terminal 23 relative to the middle portion of the principalsurface 24 and that is near the negative terminal 23. The strain gauge5B is installed at the corner 24B that is on the side of the negativeterminal 23 relative to the middle portion of the principal surface 24and that is far from the negative terminal 23. The strain gauge 5C isinstalled at the corner 24C that is on the side of the positive terminal22 relative to the middle portion of the principal surface 24 and thatis far from the positive terminal 22. The strain gauge 5D is installedat the corner 24D that is on the side of the positive terminal 22relative to the middle portion of the principal surface 24 and that isnear the positive terminal 22.

Strains S1 to S4 (hereafter also denoted as measured values S1 to S4 atthe strain gauges 5A to 5D) detected by the respective strain gauges 5Ato 5D each correspond to a slight mechanical change that is made inaccordance with a force (load) applied to a corresponding installationportion. When the volume of the lithium-ion battery 2 expands, a load isapplied to the housing 21 of the lithium-ion battery 2, from the insideto the outside of the housing, and thus pressure of the surface of thehousing 21 is increased. When the surface pressure of the lithium-ionbattery 2 is increased, the strains S1 to S4 detected by the straingauges 5A to 5D are increased accordingly. The degree of volumeexpansion at the respective positions of the battery varies inaccordance with the extent of degradation at the position. Adistribution for the volume expansion is shown on the principal surface24, as represented by dotted lines in FIG. 2. At the local position G ofdegradation where the largest degradation of the battery occurs, thedegree of the volume expansion of the battery also becomes maximal. Thestrains S1 to S4 at the strain gauges 5A to 5D that are disposed at thefour corners of the principal surface 24 each differ depending on theposition of the principal surface 24 that is such a maximum expansionposition G (i.e., local position of degradation) where volume expansionis maximal.

Now, as illustrated in FIG. 2, a two-dimensional coordinate system inwhich the center of the principal surface 24 is given as the origin O,an extension direction of one side on which the positive terminal 22 andthe negative terminal 23 are provided is the x-axis direction, and adirection perpendicular to the x-axis direction is the y-axis direction,is considered. In the two-dimensional coordinate system, the straingauge 5A is disposed in a first quadrant, the strain gauge 5D isdisposed in a second quadrant, the strain gauge 5C is disposed in athird quadrant, and the strain gauge 5B is disposed in a fourthquadrant.

In such a two-dimensional coordinate system, for example, when themaximum expansion position G is at the origin O, each of the straingauges 5A to 5D is approximately at the same distance from the origin O.Thus, the gauges 5A to 5D tend to output the respective measured valuesS1 to S4 each indicating approximately the same value. Note that afterdegradation of the battery is accelerated greatly, the maximum expansionposition G tends to be localized at the approximately middle portion ofthe principal surface 24. In contrast, when the maximum expansionposition G is situated on an x-positive direction side relative to theorigin O, the measured values S1 and S2 at the strain gauges 5A and 5Bthat are at the corners 24A and 24B near the maximum expansion positionG tend to be greater than the measured values S3 and S4 at the otherstrain gauges 5C and 5D. Also, when the maximum expansion position G issituated on an x-negative direction side relative to the origin O, themeasured values S3 and S4 at the strain gauges 5C and 5D that are at thecorners 24C and 24D near the maximum expansion position G tend to begreater than the measured values S1 and S2 at the other strain gauges 5Aand 5B. The above trends are also applied with respect to the y-axisdirection. Accordingly, in the present embodiment, characteristics ofsuch measured values S1 to S4 at the strain gauges 5A to 5D are used toestimate the maximum expansion position G of the lithium-ion battery 2,i.e., the local position G of degradation.

More specifically, the degradation determining unit 42 of the controller4 uses the following equation (1) and equation (2) to calculate, for themaximum expansion position G, an X-coordinate with respect to the x-axisdirection, and a Y-axis coordinate with respect to the y-axis direction,based on the measured values S1 to S4.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{11mu} 1} \right\rbrack\mspace{650mu}} & \; \\{X = \frac{L1\left\{ {\left( {{S1} + {S2}} \right) - \left( {{S3} + {S4}} \right)} \right\}}{\sum_{i = 1}^{4}S_{i}}} & (1) \\{\left\lbrack {{Math}.\mspace{11mu} 2} \right\rbrack\mspace{644mu}} & \; \\{Y = \frac{L2\left\{ {\left( {{S1} + {S4}} \right) - \left( {{S2} + {S3}} \right)} \right\}}{\sum_{i = 1}^{4}S_{i}}} & (2)\end{matrix}$

Where, L1 is a length of the side of the principal surface 24 in thex-axis direction, and L2 is a length of the side of the principalsurface 24 in the y-axis direction.

In Equation (1) above, a difference between the sum S1+S2 of themeasured values at the two strain gauges 5A and 5B, which are arrangedon the positive side of the x-axis of the two-dimensional coordinatesystem, and the sum S3+S4 of the measured values at the two straingauges 5C and 5D, which are arranged on the negative side of the x-axis,is calculated, and a deviation amount with respect to the x-axisdirection from the origin O of the principal surface 24 is calculatedbased on the calculated difference. The calculated deviation amount withrespect to the x-axis direction is calculated as the X-coordinate of themaximum expansion position G.

The calculated difference is divided by the total sum of the measuredvalues S1 to S4 to be normalized to a numerical value near 0. Then, bymultiplying the value by the length L1 of the side of the principalsurface 24 in the x-axis direction, Equation (1) is formulated such thatthe calculated X-coordinate falls within the range between a givenx-axis position of each of the strain gauges 5A and 5B, and a givenx-axis position of each of the strain gauges 5C and 5D.

Likewise, in Equation (2) above, a difference between the sum S1+S4 ofthe measured values at the two strain gauges 5A and 5D, which arearranged on the positive side of the y-axis of the two-dimensionalcoordinate system, and the sum S2+S3 of the measured values at the twostrain gauges 5B and 5C, which are arranged on the negative side of they-axis, is calculated, and a deviation amount with respect to the y-axisdirection from the origin O of the principal surface 24 is calculatedbased on the calculated difference. The calculated deviation amount withrespect to the y-axis direction is calculated as the Y-coordinate of themaximum expansion position G.

As in Equation (1), the calculated difference is divided by the totalsum of the measured values S1 to S4 to be normalized to a numericalvalue near 0. Then, by multiplying the value by the length L2 of theside of the principal surface 24 in the y-axis direction, Equation (2)is formulated such that the calculated Y-coordinate falls within therange between a given y-axis position of each of the strain gauges 5Aand 5D, and a given y-axis position of each of the strain gauges 5B and5C.

The maximum expansion position G calculated by Equations (1) and (2)above is a position where volume expansion is maximal in a regiondefined by the strain gauges 5A to 5D. In such a case, at least fourstrain gauges 5A to 5D need to be installed. However, a configuration inwhich more than four strain gauges are installed to improve accuracy forestimation of the position G of degradation, may be used. For example,when additional four strain gauges are each disposed at an intermediateposition (around the midpoint of each of the four sides of theprincipal, surface 24) of given strain gauges among the four straingauges 5A to 5D that are installed at the four corners, the number ofsensors is doubled in a section having approximately the same size asthat defined in the case of the four strain gauges only. Accordingly,the position of degradation can be estimated more accurately.

Referring to FIG. 3, a method for determining degradation at thedegradation-determination system 1 according to the embodiment will bedescribed. FIG. 3 is a flowchart illustrating a procedure for adegradation-determination process of the lithium-ion battery 2 performedby the degradation-determination system 1 according to the embodiment.The process related with the flowchart in FIG. 3 is executed by thecontroller 4.

In step S01 (pressure-detection step), strains S1 to S4 at the fourcorners 24A to 24D of the principal surface 24 of the lithium-ionbattery 2 are respectively measured by the strain gauges 5A to 5D. Themeasured strains S1 to S4 are each output to the degradation determiningunit 42.

In step S02 (degradation-determination step), the degradationdetermining unit 42 uses Equation (1) and Equation (2) above tocalculate X and Y coordinates of the maximum expansion position G wherevolume expansion of the principal surface 24 is maximal, based on thestrain measured values S1 to S4 measured in step S01. The degradationdetermining unit 42 outputs the maximum expansion position G as thelocal position G of degradation where the largest degradation of thebattery occurs. When the process in step S02 is completed, the controlflow ends.

Note that after the local position G of degradation is calculated instep S02, control (such as utilizing of information indicating theposition of degradation, as control information in a battery managementsystem (BMS)) related with the charge may be performed by the chargecontrol unit 41, such that safety of the lithium-ion battery 2 can beimproved.

As described above, the degradation-determination system 1 according tothe present embodiment includes at least four strain gauges 5A to 5Dthat are installed on the principal surface 24 of the lithium-ionbattery 2 and each of which detects the pressure of the battery surfaceat a corresponding installation position. The degradation-determinationsystem 1 also includes the degradation determining unit 42 thatdetermines degradation of the lithium-ion battery 2 based on themeasured values S1 to S4 at the strain gauges 5A to 5D. The degradationdetermining unit 42 estimates the maximum expansion position G wherevolume expansion is maximal in a given region defined by the straingauges 5A to 5D, of the surface of the lithium-ion battery 2.

With such a configuration, based on the measured values S1 to S4 at thestrain gauges 5A to 5D, the maximum expansion position G of theprincipal surface 24 of the lithium-ion battery 2, i.e., the position Gof degradation where the largest degradation of the battery occurs, canbe locally identified. When the position G of degradation of thelithium-ion battery 2 can be locally identified, it can be utilized asbig data to be used for analyzing or the like of a mechanism foroccurrence of degradation of the lithium-ion battery 2. Further, aprocess, such as control for preventing the charging in accordance witha state of health of the lithium-ion battery 2, can be performed moreaccurately by using information indicating the identified position G ofdegradation. For this reason, safety of the lithium-ion battery 2 can beimproved, which leads to increased life of the lithium-ion battery 2.

In the present embodiment, instead of directly measuring a given strainat each position of the principal surface 24 of the battery to therebyidentify a position where the greatest strain occurs, coordinates of thelocal position G of degradation are calculated based on the measuredvalues S1 to S4 at the four strain gauges 5A to 5D that are installed atthe four corners of the principal surface 24. With such a manner, whenat least four strain gauges 5A to 5D are installed, a given position ofdegradation can be identified. Accordingly, the number of requiredsensors can be reduced.

Further, in the degradation-determination system 1 according to thepresent embodiment, coordinates G (x, y) of the maximum expansionposition are calculated using Equation (1) and Equation (2) describedabove. Thus, the local position G of degradation of the lithium-ionbattery 2 can be estimated more accurately based on informationindicating the measured values S1 to S4 at the strain gauges 5A to 5D.

Additionally, in the degradation-determination system 1 according to thepresent embodiment, the local position G of degradation of the batterysurface is estimated based on the measured values S1 to S4 at the straingauges 5A to 5D that are installed on the surface of the lithium-ionbattery 2. In such a manner, relatively inexpensive strain gauges 5A to5D are used, resulting in a low-cost way.

Moreover, in the degradation-determination system 1 according to thepresent embodiment, the strain gauges 5A to 5D are respectivelyinstalled at the four corner regions 24A to 24D of the approximatelyrectangular principal surface 24 of the lithium-ion battery 2. With sucha manner, the section in which the local position G of degradation isable to be estimated can be maximized.

The present embodiment has been described above with reference to thespecific examples. However, the present disclosure is not limited tothese specific examples. Modifications to the specific examples to whichthose skilled in the art would make design changes as appropriate arealso included within a scope the present disclosure as long as they havefeatures of the present disclosure. Elements provided in the specificexamples described above, arrangement, conditions, shape, and the likethereof are not limited to the examples and can be modified asappropriate. For the elements provided in the above-described specificexamples, a combination thereof can be modified as appropriate, unlessthere is technical inconsistency.

In the above embodiments, the configuration in which the local positionG of degradation of the battery is estimated based on the measuredvalues S1 to S4 at the strain gauges 5A to 5D that are installed on thebattery surface, is illustrated. However, when variations in surfacepressure of a given battery can be measured, another pressure detectingunit such as a pressure sensor other than the strain gauge, may be used.

Further, in the above embodiments, the configuration in which the straingauges 5A to 5D are installed at the four corner regions of theprincipal surface 24 of the lithium-ion battery 2, is illustrated.However, when at least four strain gauges are disposed, installationpositions of the strain gauges 5A to 5D may be positions other than thefour corners of the principal surface 24. The strain gauges 5A to 5D maybe also disposed on one surface (side surface or top surface) other thanthe principal surface 24, of the surfaces of the lithium-ion battery 2.

In the above embodiments, the lithium-ion battery 2 is used as anexample of a target for which degradation is determined. However,another secondary battery such as a nickel hydride battery or a leadbattery can be adopted.

This International Application claims priority to Japanese PatentApplication No. 2018-147714, filed Aug. 6, 2018, the contents of whichare incorporated herein by reference in their entirety.

REFERENCE SIGNS LIST

-   1 degradation-determination system-   2 lithium-ion battery (secondary battery)-   3 charging device-   4 controller-   5A to 5D strain gauge (pressure detecting unit)-   21 housing-   22 positive terminal-   23 negative terminal-   24 principal surface-   24A to 24D corner-   41 charge control unit-   42 degradation determining unit-   step S01 pressure-detection step-   step S02 degradation-determination step-   S1 to S4 measured value-   G maximum expansion position-   L1 side length in x-axis direction-   L2 side length in y-axis direction

1. A degradation-determination system for a secondary battery comprising: four strain gauges that are respectively installed at four corners of an approximately rectangularly shaped principal surface of one or more surfaces of the secondary battery and each of which is configured to detect pressure of a battery surface at a corresponding installation position; and a degradation determining unit configured to determine degradation of the secondary battery based on measured values at the four strain gauges, wherein the degradation determining unit is configured to estimate a maximum expansion position where volume expansion is maximal in a region defined by the four strain gauges, of the one or more surfaces of the secondary battery.
 2. The degradation-determination system for a secondary battery according to claim 1, wherein the four strain gauges are disposed on one surface of the one or more surfaces of the secondary battery, and wherein the degradation determining unit is configured to: set a two-dimensional coordinate system such that a center of the one surface is given as the origin and the four strain gauges are disposed in respective quadrants; calculate a difference between the sum of measured values at two strain gauges that are arranged on a positive side of an x-axis of the two-dimensional coordinate system and that are among the strain gauges, and the sum of measured values at two strain gauges that are arranged on a negative side of the x-axis of the two-dimensional coordinate system, and calculate a deviation amount from the center with respect to an x-axis direction, based on the difference; calculate a difference between the sum of measured values at two strain gauges that are arranged on a positive side of a y-axis of the two-dimensional coordinate system and that are among the strain gauges, and the sum of measured values at two strain gauges that are arranged on a negative side of the y-axis of the two-dimensional coordinate system, and calculate a deviation amount from the center with respect to a y-axis direction, based on the difference; and calculate coordinates of the maximum expansion position, based on the deviation amount with respect to the x-axis direction and the deviation amount with respect to the y-axis direction.
 3. (canceled)
 4. (canceled)
 5. The degradation-determination system for a secondary battery according to claim 1, wherein the secondary battery is a lithium-ion battery.
 6. A method for determining degradation of a secondary battery, the method comprising: detecting pressure of a battery surface at a corresponding installation position, by each of four strain gauges installed at respective four corners of an approximately rectangularly shaped principal surface of one or more surfaces of the secondary battery; and determining degradation of the secondary battery based on measured values at the four strain gauges, wherein in the determining of the degradation, a maximum expansion position where volume expansion is maximal in a region defined by the four strain gauges, of the one or more surfaces of the secondary battery, is estimated.
 7. The degradation-determination system for a secondary battery according to claim 2, wherein the secondary battery is a lithium-ion battery. 